1. Field of the Invention
The present invention relates to a hybrid waveguide sensor which can reduce a propagation loss and can enhance sensitivity.
2. Description of the Related Art
As the environment changes and is polluted, a need for environmental sensors, bio-sensors, and gas sensors gradually increases. Particularly, there is a need for such a sensor that is much more sensitive and is small enough to carry.
The development of sensors has been continued for dozens of years. Among a variety of techniques which have contributed to the development of sensors, the technique for optical sensors have been significantly developed. Particularly, sensors using surface plasmons (SP) have more excellent sensitivity than the other sensors. Therefore, the sensors using surface plasmons are being actively manufactured.
Surface plasmons are charge-density oscillations which propagate along an interface between materials with dielectric constants having a reverse sign. In general, surface plasmons exist at the interface between metal having the negative sign and a dielectric having the positive sign, and can be excited by electrons accelerated at high speed and optical waves. Electromagnetic waves which are coupled to surface plasmons so as to propagate are referred to as surface plasmon polarions (hereinafter, referred to as “SPP”).
Since the wave vector of the surface plasmon is larger than those of surrounding materials, the SPP is bound to a metal surface. Therefore, the interface between metal and a dielectric can be considered as a two-dimensional optical waveguide.
In view of the optical waveguide, SPPs to be generated at the interface between metal and a dielectric are effectively bound to the metal surface, while a propagation distance thereof is as short as dozens of mm in a visible-ray region. However, when the thickness of metal is limited to several nm to dozens of nm such that SPPs propagating at the interface is coupled to each other, long-range transmission of light can be implemented. These are referred to as long-range surface plasmon polariton (LR-SPP) modes. The field profile of the LR-SPP modes is widely distributed in a dielectric around a thin metal film. Therefore, a propagation loss of light is small, and a coupling characteristic with optical fiber is excellent. Accordingly, the LR-SPP modes are applied to various optical elements.
Hereinafter, a conventional metal waveguide using LR-SPP modes and a conventional waveguide sensor using the same will be described with reference to FIGS. 1 and 2.
FIG. 1 is a sectional view of a conventional metal waveguide using LR-SPP modes.
Referring to FIG. 1, the conventional metal waveguide includes a core composed of metal 110 of which the cross-section has a predetermined thickness and width and which is formed in a strip shape; and a dielectric 120 surrounding the core. The entire structure of the waveguide may be also formed in a strip shape.
Such a conventional waveguide couples light to the metal 110 such that SPP waves propagate in the longitudinal direction of the metal 110. In this case, since the metal 110 has a predetermined thickness and width of cross-section in a direction perpendicular to the propagation direction of the SPP waves, the SPP waves can be two-dimensionally bound, which makes it possible to implement a waveguide using LR-SPP modes.
FIG. 2 is a plan view of a conventional waveguide sensor using the waveguide shown in FIG. 1.
As shown in FIG. 2, the waveguide shown in FIG. 1 is constructed as an MZI (Mach-Zehnder Interferometer) type waveguide. Then, a reference signal is applied to one arm 10, and a substance to be measured is caused to interact with the waveguide in the second arm 20 such that a phase shift generated by receiving a modulated signal is detected. Then, it is possible to sense a type or concentration of the substance.
When the entire core of the above-described waveguide is formed of metal such that the waveguide can be utilized as a sensor using the reaction of a sensing substance with the surface of the metal, high sensitivity can be obtained because of the reaction of the metal. However, an overall propagation loss is large.
Meanwhile, although not shown, a dielectric waveguide sensor composed of a general dielectric has little propagation loss of light. Therefore, the dielectric waveguide sensor has a considerable advantage in maintaining a basic structure as a sensor. However, the dielectric waveguide sensor has lower sensitivity than the metal waveguide sensor.
Therefore, in this technical field, there is a demand for reducing a propagation loss and enhancing sensitivity.